Yes, Portal for ArcGIS supports join and relates. Portal for ArcGIS has the Join Features function as an analysis tool. Refer to Portal for ArcGIS: Join Features for more information.
Note: Performing analysis is currently only supported in Map Viewer Classic. Refer to Portal for ArcGIS: Perform analysis for more information.
Follow the steps below to use the Join Features tool:
The related records are represented as columns with italicized headers on the right side of the attribute table. In this example, the Boundaries column displays the related records. Refer to Portal for ArcGIS: Edit attributes and related records for more information.